1. Field of the Invention
The present invention relates generally to fiber-reinforced composite structures and the molding materials that are used to make such structures. More particularly, the present invention involves the use of unidirectional prepreg tape to form pre-plied, multi-directional, continuous fiber laminates that are suitable for molding complex three dimensional fiber-reinforced composite structures via near-net shaped pre-forming.
2. Description of Related Art
Fiber-reinforced composite structures typically include a resin matrix and fibers as the two principal components. These structures are well-suited for use in demanding environments, such as in the field of aerospace, where a combination of high strength and light weight is important.
Pre-impregnated composite material (prepreg) is used widely in the manufacture of composite parts and structures. Prepreg is a combination of uncured resin matrix and fiber reinforcement that is ready for molding and curing into the final composite part. By pre-impregnating the fiber reinforcement with resin, the manufacturer can carefully control the amount and location of resin that is impregnated into the fiber network and ensure that the resin is distributed in the network as desired. Prepreg is a preferred material for use in manufacturing load-bearing structural parts and particularly load-bearing aircraft parts that are used in wings, fuselages, bulkheads and control surfaces. It is important that these parts have sufficient strength, damage tolerance and other requirements that are routinely established for such parts.
Unidirectional (UD) tape is a common form of prepreg. The fibers in unidirectional tape are continuous fibers that extend parallel to each other. The fibers are typically in the form of bundles of numerous individual fibers or filaments that are referred to as a “tows”. The unidirectional fibers are impregnated with a carefully controlled amount of uncured resin. The UD prepreg is typically placed between protective layers to form the UD final tape that is rolled up for storage or transport to the manufacturing facility. The width of UD tape typically ranges from less than one inch to a foot or more.
Unidirectional tape is not well-suited for use as a molding compound for forming complex three dimensional structures using compression molding techniques. The parallel orientation and continuous nature of the fibers in the UD tape cause fiber bunching or bridging when the UD tape is forced to fit the features of the complex part. As a result, the manufacture of complex three dimensional parts using UD tape has been limited to a laborious process where individual plies of UD tape are applied directly to a three dimensional mold, which is subsequently processed in an autoclave or other molding apparatus. This lay-up procedure using UD tape tends to be a long and costly process.
Molding compounds that have been found to be suitable for compression molding complex parts commonly employ randomly oriented short fibers that more easily fit the features of the part. However, the use of such short fibers, when assembled into a random mat, introduces local weight variation. The weight variation creates several problems. For example, it contributes to the complexity of the ply design, which has to accommodate for all of the possible total weight outcomes when assembling several highly variable plies for a specific part geometry. The local weight variations in the random mat of short fibers also contribute to irregularities during molding, because areas of low weight are compensated for by areas of high weight. This compensation process differs unpredictably from one molded part to the next and also differs between different features of a given part. As a result, it is difficult for the designer to predict and determine if the molding compound design will be adequate for forming the desired part.
In view of the above, there is a continuing need to provide prepreg molding methods that are suitable for use in compression molding fiber-reinforced composite structures that have a relatively complex shape. The need for such method is especially important in those situations where the strength of the part is a prime consideration.